fix(gitlab): fetch merge request heads using GitLab refs

[saphid]

Fixes #6448

Problem

Checking out a GitLab merge request in a separate worktree fails with "Could not prepare the pull request checkout" whenever the MR's source-repository metadata is unavailable. That happens with an unauthenticated or limited glab view, and a user report shows authenticated glab 1.102.0 and 1.121.0 also omit source_project for same-project MRs on self-managed GitLab. In that case the server fetches refs/pull/<n>/head, which GitLab does not publish; GitLab uses refs/merge-requests/<n>/head. For the same reason, an existing MR worktree whose head cannot be resolved through its upstream can stay on old commits and report that it is not on the MR head.

Current main already falls back to fetching an open same-repository head by branch name when the head-ref fetch fails (added for Azure DevOps). That covers some GitLab cases, but not closed or merged MRs, MRs whose source branch was deleted, or refreshing an existing MR worktree.

This is a small, focused fix under the contribution exception: worktree checkout is an existing feature, #6448 reports the same failure and traces it to the hardcoded refspec, and other hosts keep their behaviour. The broader earlier attempt, #6532, is closed unmerged.

Change

• SourceControlProvider gets an optional changeRequestHeadRef(number). Absent means refs/pull/<n>/head, so GitHub, Forgejo, Azure DevOps and Bitbucket are unchanged.
• The GitLab provider returns refs/merge-requests/<n>/head.
• GitManager reads the ref from the provider that resolved the change request and passes it to both materialization fetches and to the head read used when an existing worktree is reused. Main's open same-repository branch fallback stays in place behind it.
• The git driver uses the given ref, defaulting to refs/pull/<n>/head. The RPC contract does not change.

Verification

At head f7d32c7 (current main merged in, Linux, git from the host):

• vp test run apps/server/src/git/GitManager.test.ts apps/server/src/vcs/GitVcsDriverCore.test.ts -t 'materializes and refreshes GitLab|without moving the checkout'
  • With main's versions of the non-test files: 2 failed (GitLab MR worktree materialize + refresh; driver fetch from refs/merge-requests/42/head), 1 passed (GitHub default).
  • With this PR: 3 passed.
• Full GitManager.test.ts + GitVcsDriverCore.test.ts: 292 passed, 1 failed. The failure is review diff previews > preserves renames, unusual paths, modes, and binary statistics, which fails the same way on unmodified main in this environment; this PR does not touch that code.
• Server tsc --noEmit: exit 0, no errors. vp lint on the changed files: no errors (two warnings on unchanged lines).

Tests added:

• GitManager.test.ts mirrors glab without source-repository metadata: it creates an MR worktree from a bare remote's refs/merge-requests/90/head, advances that ref, prepares the MR again, and asserts the same worktree moves to the new head.
• GitVcsDriverCore.test.ts fetches a GitLab head ref and the GitHub default against a real remote and checks the current checkout does not move.

The public remote shows the ref mismatch directly (re-checked 2026-10-14):

git ls-remote https://gitlab.com/gitlab-org/gitlab-test.git refs/merge-requests/68/head refs/pull/68/head
094f26019a639c319fc9e59382f090c1bcaeeec8	refs/merge-requests/68/head

Real-client evidence (earlier head)

Real web client against the dev server on loopback, isolated T3 state, fresh clone of https://gitlab.com/gitlab-org/gitlab-test.git, opening the closed MR !68 and choosing Check out → In a separate worktree. glab was not logged in; nothing was mocked. Before ran on main c5a0c78b7c, after on this branch rebased onto it. MR !68 is closed, so main's open-branch fallback does not apply to it and the before result still holds on current main.

Before (main): "Could not prepare the pull request checkout"; no MR worktree is created.

After: "Checked out"; the new worktree's HEAD is 094f26019a639c319fc9e59382f090c1bcaeeec8.

Recording of the after run at 3× (real-time MP4):

Surfaces

• Web: real client, above.
• Desktop: bundles the same server; no shell or IPC change. Desktop shell not exercised.
• Mobile: benefits from the same server fix: the thread outbox (apps/mobile/src/state/use-thread-outbox-drain.ts) calls git.preparePullRequestThread. The native client was not exercised.
• Local / relay / tunnel: the fix runs in the server's git fetch and the wire contract is unchanged; only local loopback was exercised.
• Agent providers: not involved; this is source-control host behaviour.

Not checked

• Self-managed GitLab (only gitlab.com); ref selection depends on the resolved provider kind, not the hostname.
• A force-pushed (non-descendant) MR head on a worktree with no upstream: existing behaviour applies, the checkout is kept and reported as not on the MR head.

GPT-6.1 Sol, Claude Opus 5.5 and GPT-6 Astra via T3 Code. Merged current main and re-verified with Claude Opus 5.5 (Claude Code) via T3 Code; review GPT-6.1 Sol.

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Reviewing files that changed from the base of the PR and between 857e308 and 940c1bd.

📒 Files selected for processing (7)

• apps/server/src/git/GitManager.test.ts
• apps/server/src/git/GitManager.ts
• apps/server/src/vcs/GitVcsDriver.ts
• apps/server/src/vcs/GitVcsDriverCore.test.ts
• apps/server/src/vcs/GitVcsDriverCore.ts
• packages/source-control-core/src/server/SourceControlProvider.ts
• packages/source-control-gitlab/src/server/GitLabSourceControlProvider.ts

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📝 Walkthrough

📝 Walkthrough

Walkthrough

Source-control providers can supply a host-specific pull-request head ref. Git fetch operations use that ref or default to refs/pull/<number>/head. GitManager passes the provider’s ref during pull-request preparation and worktree refresh.

Changes

Host-specific pull-request head fetching

Layer / File(s)	Summary
Define and fetch the head ref packages/source-control-core/src/server/SourceControlProvider.ts, packages/source-control-gitlab/src/server/GitLabSourceControlProvider.ts, apps/server/src/vcs/GitVcsDriver.ts, apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	The provider interface adds an optional callback for the change-request head ref. GitLab returns refs/merge-requests/<number>/head. Git fetch operations use a supplied headRef or default to refs/pull/<number>/head. Tests cover both ref formats and verify that fetches do not move HEAD.
Pass the head ref through preparation apps/server/src/git/GitManager.ts, apps/server/src/git/GitManager.test.ts	GitManager passes the provider-supplied ref through local-checkout and worktree preparation, including reused-worktree refresh. A test checks initial GitLab worktree preparation and refresh after the merge-request head changes.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant GitManager
  participant SourceControlProvider
  participant GitVcsDriverCore
  participant GitRemote
  GitManager->>SourceControlProvider: Get change-request head ref
  GitManager->>GitVcsDriverCore: Fetch branch or head commit using headRef
  GitVcsDriverCore->>GitRemote: Fetch selected head ref

Loading

Possibly related PRs

• pingdotgg/t3code#6532: Both changes pass host-specific head refs through GitManager to Git fetch operations. That PR also covers Bitbucket and Azure DevOps fallback cases.

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 940c1

GitLab merge requests can now be checked out and refreshed using their published head ref, and other providers keep their existing behavior. No merge-blocking risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 940c1

This is a bounded change to repository-head selection. The implemented GitLab path retains existing remote, credential and worktree safeguards. No introduced security issue was identified, but custom-provider callers and interrupted or concurrent operations were not fully validated.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The reviewed path affects branch materialization and refresh in the selected repository/worktree under its existing Git credential context. The new capability does not select a remote URL or confer additional credentials. MR-controlled commits remain repository content; no new execution or service-wide authority path was identified in the compared changes.

Trust Boundaries and Controls

• observed — Git execution uses argv arrays rather than shell-command interpolation. With the inspected GitLab ref producer, branch fetching retains the constructed local branch destination, while head-commit fetching uses FETCH_HEAD without directly updating the checked-out branch. The low-level headRef interface itself has no runtime plain-ref validation, so argv separation alone does not establish safety for arbitrary future refspec or option-like inputs.

Resilience and Maintainability Implications

• observed — A reused worktree without an upstream still refuses a non-descendant target when its current commit cannot match the captured upstream commit. This can leave the checkout stale after a force-push, but the same guard and null-upstream behavior exist at the merge base; it is not an introduced PR concern.

Hardening Proposals

• proposed — Before allowing externally configured ref producers, document this capability as a trusted plain source-ref contract and validate it at the Git boundary to reject option-like values and destination-bearing refspecs. This is future hardening, not an observed exploit in the GitLab implementation.

Pre-merge checks: 6

Passed checks: 6 passed

Check name	Status	Explanation
Linked Issues check		The description links issue #6448, explains the matching failure, and documents why this focused fix addresses the issue despite the issue being closed.
Out of Scope Changes check		The changes are limited to GitLab head-ref resolution, shared provider plumbing, Git fetch behavior, and focused regression tests. The described scope matches the implementation.
Linked Issues check		Issue #6448 is closed and completed. It supplies historical context only, so no active coding requirements apply. The reported changes address the historical GitLab checkout and refresh failure, but t…
Out of Scope Changes check		The changed files add provider-specific ref selection, pass that ref through GitManager and the Git driver, and add focused regression tests. These changes support GitLab merge-request checkout and re…
Title check		The title clearly and concisely identifies the main change: fetching GitLab merge request heads through GitLab-specific refs.
Description check		The description includes the required Problem, Change, Scope and approval, and Verification sections. It explains the failure, implementation, approval exception, focused tests, observed results, limi…

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[macroscopeapp]

Approvability

Verdict: Approved at 693310f

Macroscope's review found this PR approvable — This is a narrowly scoped GitLab bug fix that routes existing merge-request checkout and refresh operations to GitLab’s published head ref, while preserving the prior default for other providers. Regression tests cover both the GitLab-specific and fallback paths, with no schema, deployment, security, billing, or product-default changes.

No code changes detected at f7d32c7. Prior analysis still applies.

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Dismissing prior approval to re-evaluate ee50a62

Dismissing prior approval to re-evaluate 3080acd

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Actionable comments posted: 1

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Inline comments:
Review comments at @apps/server/src/git/GitManager.ts:
- Line 2455: Update the GitManager worktree refresh flow to persist the accepted
MR head per worktree, advance it after every successful refresh, and reset a
non-descendant MR head only when the current HEAD still matches that recorded
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📒 Files selected for processing (5)

• apps/server/src/git/GitManager.test.ts
• apps/server/src/git/GitManager.ts
• apps/server/src/vcs/GitVcsDriver.ts
• apps/server/src/vcs/GitVcsDriverCore.test.ts
• apps/server/src/vcs/GitVcsDriverCore.ts

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[saphid]

Two notes on the CodeRabbit summary for the rebased head:

• The "Merge Risk: Moderate / fix this refresh path" note restates the inline finding at GitManager.ts:2455, which CodeRabbit later withdrew in that thread. The force-push case with no upstream uses existing behaviour this PR doesn't change: the checkout is kept and reported as not on the MR head. The PR description now lists that case under "Not checked".
• Docstring coverage: the two functions counted are module-private helpers (pullRequestHeadRef, and toPullRequestHeadRemoteInfo, which has no doc comment on main either). The repo doesn't docstring private helpers. The only public method touched, GitVcsDriver.fetchPullRequestHeadCommit, has an updated doc comment. Leaving as is.

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-03	Julius	Discord DM

_{Logged so this PR shows when a maintainer was asked to review it.}

Dismissing prior approval to re-evaluate 962307b

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-06	Julius	Discord DM

_{Logged so this PR shows when a maintainer was asked to review it.}

[w3z315]

Data point for the "Not checked" items: self-managed GitLab with an authenticated glab 1.102.0, same-project MR, nightly 0.0.46-nightly.20261008. Worktree checkout fails with Failed to materialize pull request #285 …; the trace shows both the primary and the fallback attempt fetching refs/pull/285/head, while the remote only has refs/merge-requests/285/head.

So this isn't limited to unauthenticated or limited views: glab mr view --output json didn't include source_project with an authenticated 1.102.0 or 1.121.0 either, so every same-project MR takes this path.

Side note: #6448 was closed as completed via #7174, which was closed unmerged.

Workaround until this lands: create the worktree for the MR branch with git worktree add. preparePullRequestThread then reuses it.

[saphid]

Thanks, this is very useful. It confirms the failure isn't limited to unauthenticated or limited views: with authenticated glab 1.102.0 and 1.121.0, mr view --output json still omits source_project for same-project MRs, so every same-project MR falls back to refs/pull/<n>/head, which GitLab doesn't publish. This PR fetches refs/merge-requests/<n>/head in that case. I've updated the description to cite your report.

Your git worktree add workaround matches what the fix relies on: preparePullRequestThread reuses an existing worktree for the MR branch.

Noted on #6448/#7174, too. This PR is the open fix for that path.

Dismissing prior approval to re-evaluate 940c1bd

Dismissing prior approval to re-evaluate 693310f

[saphid]

@w3z315 thanks for the self-managed report. It matches what this PR fixes: when glab returns no source_project, the server falls back to fetching the change request's head ref, and on GitLab that has to be refs/merge-requests/<n>/head rather than refs/pull/<n>/head.

Current main does fetch an open same-project MR by branch name when the head ref fetch fails, so some of those checkouts now work without this PR. It still fails for closed or merged MRs, MRs whose source branch was deleted, and refreshing an existing MR worktree. I merged current main into the branch (f7d32c7) and re-ran the new GitLab regression tests against main's sources: they still fail there and pass on this head. The fix has not been run on your instance.